The research proposed in this Phase I grant application will lead to the development of a very high-resolution scintillation detector array for positron emission tomography (PET) imaging. Two alternative schemes will be investigated to improve the image resolution and quantitative accuracy of PET. The first will be an advance to the current "block" detector concept based on an 8x8 array of Bismuth Germanate (BGO), or Lead Sulfate (PbSO4) scintillation crystals coupled through a light guide to four photomultiplier tubes. The crystals will be spaced 3mm apart both axially and transaxially and should provide resolution of approximately 2.5mm in all directions. The second detector configuration will use similar sized array of scintillators but coupled to a grid of silicon photodiodes to determine the interaction detector, with a single large PMT to provide the timing signals. Prototype detectors will be cut from solid blocks of scintillator and tested for the photoelectron yield and event positioning characteristics. The light guide will be adjusted to provide maximum differentiation in the positioning signals between the various crystals of the array while maintaining high light output. Once the lightguide designs have been determined, more detailed measurements will be performed to characterize the positioning accuracy, time resolution, energy resolution, and efficiency. This research should produce the highest resolution PET modular detector design based on four photomultipliers.